1) Field of the Invention
The field of the present invention relates to reading symbols such as bar codes, and more particularly to a bar code or symbol reader having integrated optical and signal processing circuitry.
2) Background
Currently available bar code readers typically have from ten to fifty integrated circuits, as well as several dozen mechanical and optical parts. The market demands, however, are for dramatically decreased cost and size of bar code readers, without sacrificing reliability or performance. A reduction in the number of parts used in bar code readers would help meet these demands.
Previous attempts to reduce, through various levels of integration, the number of parts in visible laser-based scanners have been fraught with difficulties. Visible laser diodes, which are typically made using AlGaInP material, cannot be directly integrated on a silicon substrate, which is where at least some part of the electronic or signal processing circuitry is likely to be resident. Hybrid approaches, whereby the laser diode is mounted to a thermally-conductive pad which in turn is mounted on a silicon substrate, have been demonstrated by Sony Corporation (for example, in their laser coupler used in compact disc products). In this type of construction, the performance of the electronic circuitry on the silicon substrate suffers because of the large thermal gradient across the die. Performance is also adversely affected by photo-recombination from stray laser light, causing an increase in noise levels. Moreover, reliability is also relatively poor, as with many laser-based scanner products, because of huge power density at the laser junction.
Some bar code readers use charge-coupled devices (CCDs) as a substitute for a laser diode. Integration of circuit components in these systems, however, is also difficult, primarily because CCD chips typically require highly-specialized metal-oxide semi-conductor (MOS) processes which are poorly suited for fabrication of other types of circuitry. For example, fabrication of high-transconductance field-effect transistors (FETs), generally needed in the construction of an operational amplifier, is not practical using the same MOS processes required of most CCD chips. Further, fabrication costs for this specialized process, per unit of silicon area, are considerably higher than those for conventional bipolar and CMOS processes, largely because very high silicon purity is needed to achieve reasonable charge transfer efficiency. Thus, integration of circuitry in a CCD-based bar code reader is difficult and, even if possible, would be relatively costly from a manufacturing standpoint.
The present inventors have therefore determined that it would be advantageous to provide a bar code or symbol reader having circuitry that is relatively easy to integrate, yet not prohibitively expensive to manufacture. It would further be advantageous to provide a bar code reader having a reduced number of parts, and, in addition, to provide a reduced-size bar code reader incorporating integrated optical and signal processing circuitry.